Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al-Mg)-alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11', a cobalt (Co)-base alloy magnetic layer 12, 12', a protective overcoat 13, 13', typically containing carbon, and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11A'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective overcoat 13, 13', typically containing carbon, are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture, which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective layer to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat, typically comprising carbon, increases friction between the head and disk, thereby causing catastrophic drive failure. Excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and catastrophic failure occurs. Accordingly, the lubricant thickness must be optimized for stiction and friction.
A significant factor in the performance of a lubricant topcoat is the amount of lubricant which tightly adheres to the magnetic recording media. The amount of adhering lubricant is described by the "bonded lube ratio" which is the ratio of the amount of lubricant directly bonded to the magnetic recording media vis-a-vis the total amount of originally applied lubricant. The portion of lubricant which is not tightly bound and easily removed by immersion of the recording media in a solvent is referred to as the "mobile lubricant".
Another significant factor in the performance of a lubricant topcoat is the ability of the lubricant to resist decomposition over time, particularly decomposition by acid catalysis. For example, lubricants that resist catalytic cleavage by Lewis acids provide improved tribology under stress conditions.
Despite the importance of lubricity in recording media, few commercial lubricants are available that satisfy the demanding criteria of a lubricant topcoat. Typical conventional lubricants, such as perfluoroalkylpolyether fluids such as PFPE-1, PFPE-2, PFPE-3, PFPE-4, do not have polar end-groups which would permit lube bonding or a high bonded lube ratio. Significantly, many conventional lubricants contain difluoroformyl (--CF.sub.2 O--) linkages, i.e. repeating units with only one carbon atom, throughout the main chain of the polymer. It is know that formyl and difluoroformyl linkages undergo acid catalyzed cleavage resulting in decomposition of the polymeric chain over time. (see e.g. T. Del Pesco Handbook of Lubrication and Tribology (1994) Vol. III pages 287-303, Booser, E.R. ed., CRC Press, Boca Raton, FL). Thus, it is believed that lubricants with formyl or halosubstituted formyl linkages compromise the integrity of the lubricant over time resulting in poor performance.
Two commercially available lubricants with functional end-groups are ZDOL.RTM. (Ausimont USA, Thorofare, N.J.) and Demnum SA (Nagase &Co., LTD.). These compounds have the following chemical structure:
ZDOL.RTM. HOCH.sub.2 CF.sub.2 O (CF.sub.2 CF.sub.2 O).sub.a (CF.sub.2 O).sub.b CF.sub.2 CH.sub.2 OH PA1 Demnum SA F (CF.sub.2 CF.sub.2 CF.sub.2 O).sub.c CF.sub.2 CF.sub.2 CH.sub.2 OH PA1 wherein PA1 (a) protecting a hydrocarbon polyether comprising at least five C.sub.2-10 alkyl repeating groups and having at least one hydroxyl end group by reacting the hydrocarbon polyether with a carboxylic acid chloride to form a polyether ester; PA1 (b) exposing the polyether ester to fluorine gas for a sufficient time and in a sufficient amount of the fluorine gas to fluorinate the polyether ester to produce a fluoropolyether ester; PA1 (c) adding an alcohol to the fluoropolyether ester to generate a second fluoropolyether ester; and PA1 (d) reducing the second fluoropolyether methylester with a reducing agent to generate a fluoropolyether alcohol.
However, ZDOL.RTM. contains difluoroformyl linkages which suffers from the same deficiencies described above and Demnum SA has only one polar end group. Moreover, neither lubricant exhibits a sufficiently high bonded lube ratio preferable for a lubricant topcoat.
The degree of direct bonding or bonded lube ratio is dependent upon the particular material employed for the protective overcoat and the lubricant end-group. Desirably, the bonded lube ratio should be controllable to realize a meaningful improvement in stiction and wear performance of the resulting magnetic recording medium.
In view of the criticality of the lubricant topcoat, there is a continuing need for improved bonding of the lubricant to the magnetic recording media, particularly to a protective carbon overcoat. There is also a need for lubricants for use as topcoats in the manufacture of recording media with improved resistance to degradation and improved tribology under stress conditions.